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Solar System
just like planets, while others— including our Moon—may have been the result of a planet colliding with the Earth and sending debris into space, which then gathered again as a moon.
Giant impacts during the early stages of the Solar System may well have sent some planets careening off on different orbits. This could explain why Uranus spins on its back, why Venus spins the opposite way from the other planets, and why each of the planets has a differently shaped orbit.
Structure
The Solar System contains nine main orbiting worlds called planets,
with their 61 moons, all orbiting the Sun. The planets lie far apart from each other, orbiting in different ways and at different speeds, and they spin on their
axes at different rates. The inner planets—Mercury, Venus, Earth, and Mars—are rocky worlds, while most of the outer giant planets—Jupiter, Saturn, Uranus, and Neptune—are mainly made
of gases and liquids. Pluto, the outermost of the planets, is a small rocky world. The Solar System, however, extends far beyond Pluto. (See also: Kuiper belt.)
The size of the Solar System
The enormity of the Solar System can be judged by the time a particle of light (a photon) leaving the Sun and traveling at about one billion km an hour, takes to reach each planet. Light from the Sun reaches the nearest planet, Mercury, in
just three minutes, flies past the Earth after eight minutes, and takes five and a half hours to reach the outermost planet, Pluto. From Pluto the Sun appears as just a disk of light set in the black starry sky, only a bit brighter than the other stars.
Pluto
Neptune
Jupiter
Uranus
Saturn
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